They just seem way too easy to gain. You said it yourself, Zerabil got 3 levels in a single day, it should've taken years of training and experiences imo. It should take all heroes a huge amount of time or an exceptional dedication to their respective areas to level up. Kuro for example is 46 years old at the moment and can beat a group of 5 soldiers in a fist fight. After 40 years of [i]daily training[/i].

Because why would a God create a Hero and level them up himself instead of creating an avatar, which is arguably a ton more powerful than any common hero?